Wood Composite Material Containing Strands of Differing Densities

ABSTRACT

Disclosed is a wood composite material comprising: wood strands disposed in surface layers and one or more core layers, whereby an intermediate density divides the wood strands so that the two surface layers of strands are more dense than the intermediate density, and the one or more core layers of the strands are less dense than the intermediate density, and wherein the core layer contains at least about 70 wt % of the strands that are less dense than the intermediate density; and the surface layers contain at least about 70 wt % of the strands that are more dense than the intermediate density.

BACKGROUND OF THE INVENTION

Wood is a common material used to construct doors and otherarchitectural building elements. Even today, after the development ofseveral new species of composite materials, wood remains one of the mostwidely-used structural materials because of its excellent strength andstiffness, pleasing aesthetics, good insulation properties and easyworkability.

However, in recent years the cost of solid timber wood has increaseddramatically as its supply shrinks due to the gradual depletion ofold-growth and virgin forests. It is particularly expensive tomanufacture doors from such material because typically less than half ofharvested timber wood is converted to natural solid wood lumber, theremainder being discarded as scrap.

Accordingly, because of both the cost of high-grade timber wood as wellas a heightened emphasis on conserving natural resources, wood-basedalternatives to natural solid wood lumber have been developed that makemore efficient use of harvested wood and reduce the amount of wooddiscarded as scrap. Plywood, particle board and oriented strand board(“OSB”) are examples of wood-based composite alternatives to naturalsolid wood lumber that have replaced natural solid wood lumber in manystructural applications in the last seventy-five years. These wood-basedcomposites not only use the available supply of timber wood moreefficiently, but they can also be formed from lower-grade wood species,and even from wood wastes.

However, decreasing wood supplies and lower wood quality are puttingincreasing stress on the quality of and raw material prices for OSB.Wood species often must be mixed in the process in order to have enoughwood to make continuing supply of composite panels. If these species areof differing densities, there are problems with the mixing of them inthe panel. For example, if a clump of one species or the other were toform in the panel due to inadequate mixing in the process, then the areaof the clumped species would have a density that is either higher orlower than the overall average density of the panel. This could causevisual and structural problems with the panel since lower densityclumped material may form a weak spot and high density clumped materialmay cause a stronger and harder area which may be harder to sand. Giventhe foregoing, there is a continuing need for a wood composite materialthat can address these inadequacies. Notably this wood compositematerial would have superior or comparable performance to solid woodlumber while being lighter (lower density) than conventionalmixed-species OSB materials, have a more uniform appearance and moreconsistent physical properties. Additionally, this wood compositematerial would incorporate to some extent fibers harvested fromlignocellulosic species that are faster growing than those species whichare conventionally used for wood composite materials.

Examples of such faster growing lignocellulosic species include treespecies from the genus Paulownia, as well as trees grown on managed(irrigation and fertilization) plantations. Examples of non-treelignocellulosic materials include annual and perennial grasses such aswheat straw and the various species of bamboo. These can be grown andharvested very quickly after planting, and can give a more frequentharvest schedule than that of typical tree species. Additionally, thesespecies would be mixed into a wood composite material in such a way thatthe strengths of the materials would be emphasized while the weaknessesof the materials would be minimized for the final product. Mixing asmall amount of a superior material into a wood composite in this mannerwould serve to maximize the value of the superior material while at thesame time conserving it, especially if the material was in short supplyor was expensive to procure.

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BRIEF SUMMARY OF THE INVENTION

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DETAILED DESCRIPTION OF THE INVENTION

All parts, percentages and ratios used herein are expressed by weightunless otherwise specified. All documents cited herein are incorporatedby reference.

As used herein, “wood” is intended to mean a cellular structure, havingcell walls composed of cellulose and hemicellulose fibers bondedtogether by lignin polymer. It should further be noted that the term“wood” encompasses lignocellulosic material generally.

By “wood composite material” it is meant a composite material thatcomprises wood and one or more wood composite additives, such asadhesives or waxes. The wood is typically in the form of veneers,flakes, strands, wafers, particles, and chips. Non-limiting examples ofwood composite materials include oriented strand board (“OSB”),waferboard, particle board, chipboard, medium-density fiberboard,plywood, parallel strand lumber, oriented strand lumber, and laminatedstrand lumbers. Common characteristic of the wood composite materialsare that they are composite materials comprised of strands and plyveneers binded with polymeric resin and other special additives. As usedherein, “flakes”, “strands”, “chips”, “particles”, and “wafers” areconsidered equivalent to one another and are used interchangeably. Anon-exclusive description of wood composite materials may be found inthe Supplement Volume to the Kirk-Rothmer Encyclopedia of ChemicalTechnology, pp 765-810, 6^(th) Edition.

The present invention is directed to wood composite material (especiallywood composite boards and panels) comprising a combination of core andsurface layers (for a total of three or more layers) in such a way thatthe strands in the core layer are made from a less dense wood speciesthan the strands in the surface layer.

Several factors can affect the density of wood, and consequently thewood strands made from them. Some wood species simply are heavier anddenser than other wood species. For example iron wood, oak and hickoryare all known as being very heavy wood species, which is physicallyexplained by their having smaller interior spaces in the wood lumen. Onthe other hand, less dense wood material is typically selected fromtrees that are less mature and that grow faster than other trees. Thisspeed of growth may partially be the result of specially selectingcertain genetic tree strains that are faster growing. But the speed ofgrowth is particularly the result of the trees being grown onplantations where conditions are ideal for faster growth such as regularspacing, the clearing of brush and under growth, and consistent andmanaged fertilization and water, and care and tending.

However, the drawback of using fast-growing wood species with lowerdensities and higher fraction of juvenile wood is that such wood has alower strength and stiffness. For example, low density lumber of aparticular size is not as strong as higher density lumber of the samedimensions, therefore superior or comparable strength and stiffnessperformance must be obtained by cutting the low density wood intothicker pieces or using it in combination with higher density lumbermaterials.

The boards or panels prepared according to the present invention may bemade in the form of a variety of different materials, such as wood orwood composite materials, such as oriented strand board (“OSB”). Inaddition to paulownia, these OSB panels also incorporate strands fromother suitable wood species materials including naturally occurring hardor soft woods species, singularly or mixed, whether such wood is dry(having a moisture content of between 2 wt % and 12 wt %) or green(having a moisture content of between 30 wt % and 200 wt %).

In the present invention, the lower density strands are concentrated inthe core layers and the higher density strands are concentrated in thesurface layers. In this instance, the term “layer” or “layers” is meantto refer to a section of one wood species or density group, and is notnecessarily a designation of strand orientation, although each layer maybe oriented or random. Specifically, the wood strands in the presentinvention are divided by an “intermediate density”, which divides thewood strands so that the top and bottom layers of the strands are moredense than the intermediate density, and the middle layer (or layers) ofthe strands are less dense than the intermediate density. The core layercontains at least about 70 wt % of the strands that are less dense thanthe intermediate density; and the upper and lower surface layers containat least about 70 wt % of the strands that are more dense than theintermediate density.

The process for making wood composites according to the presentinvention is straightforward. In the first step, a quantity of woodstrands is provided, the strands will typically be a mixture of two ormore wood species, each having a different density. The strands are thendivided based on an intermediate density (which can be determined invariety of ways all of which are well-known to a person of ordinaryskill in the art) so that a first group of the strands are less densethan the intermediate density, and a second group of the strands aremore dense than the intermediate density;

The strands are then distributed between the surface layers and the corelayer, so that the core layer contains at least about 70 wt % of thestrands that are less dense than the intermediate density; and the upperand lower surface layers contain at least about 70 wt % of the strandsthat are more dense than the intermediate density.

Although as used in the present application, there are no specific woodspecies to be designated as “low density” and “high density” strands, itis noted that materials like paulownia, and loblolly pine are of lowerdensity than other wood species such as hickory or oak. It is to beexpected that lower density strands such as paulownia are more likely tobe included in the core layers of the wood composite panels.

One particular consequence regarding the increased concentration oflower density strands in a wood composite is that the wood compositematerial will be less dense. For example, conventional OSB boardsmeeting PS-2 standards will have a density in the range of about 35lbs/ft³ to about 48 lbs/ft³; specifically, the density ranges from 40lbs/ft³ to 48 lbs/ft³ for southern pine, and 35 lbs lbs/ft³ to 42lbs/ft³ for Aspen. By contrast,in the present invention with theincreased concentration of lower density strands, the density of theboards will range from 20 lbs/ft³ to about 60 lbs/ft³. Of course, thehigher the fraction of lower density strands used in these mixed woodspecies composites, the lower the density of the board or panel.

Regardless of its density, the panel should have a thickness of about0.6 cm (about ¼″) to about 10.2 cm (about 4″).

Typically, the raw wood starting materials, either virgin or reclaimed,are cut into strands, wafers or flakes of desired size and shape, whichare well known to one of ordinary skill in the art. The strands arepreferably more than 2 inches long, more than 0.3 inch wide, and lessthan 0.25 inch thick. While not intended to be limited by theory, it isbelieved that longer strands, i.e., longer than about 6 inches, improvesthe final product mechanical strength by permitting better alignment. Itis also known that uniform-width strands are preferred for betterproduct quality. Uniform strand geometry allows a manufacturer tooptimize the manufacturer's process for each size of strand. Forinstance, if all the stands were 4 inches×1 inch, then the orientercould be optimized to align those strands within a single layer. Ifstrands that were 1 inch long and 0.25 inch wide were added, some ofthose could slide thru the orienters sideways. Cross-oriented strandslower the overall mechanical strength/stiffness of the product.

After the strands are cut they are dried in an oven to a moisturecontent of about 1 to 20%, preferably between 2 to 18%, more preferablyfrom 3 to about 15%, and then coated with one or more polymericthermosetting binder resins, waxes and other additives. The binder resinand the other various additives that are applied to the wood materialsare referred to herein as a coating, even though the binder andadditives may be in the form of small particles, such as atomizedparticles or solid particles, which do not form a continuous coatingupon the wood material. Conventionally, the binder, wax and any otheradditives are applied to the wood materials by one or more spraying,blending or mixing techniques, a preferred technique is to spray thewax, resin and other additives upon the wood strands as the strands aretumbled in a drum blender.

After being coated and treated with the desired coating and treatmentchemicals, these coated strands are used to form a multi-layered mat. Ina conventional process for forming a multi-layered mat, the coated woodmaterials are spread on a conveyor belt in a series of two or more,preferably three layers. Preferably, the strands are positioned on theconveyor belt as alternating layers where the “strands” in adjacentlayers are oriented generally perpendicular to each other, but it isalso understood by those skilled in the art that the products made fromthis process could have the strands aligned all in the same direction orrandomly without a particular alignment.

Various polymeric resins, preferably thermosetting resins, may beemployed as binders for the wood flakes or strands. Suitable polymericbinders include isocyanate resin, urea-formaldehyde, phenolformaldehyde, melamine formaldehyde (“MUF”) and the co-polymers thereof.Isocyanates are the preferred binders, and preferably the isocyanatesare selected from the diphenylmethane-p,p′-diisocyanate group ofpolymers, which have NCO— functional groups that can react with otherorganic groups to form polymer groups such as polyurea, —NCON—, andpolyurethane, —NCOO—. 4,4-diphenyl-methane diisocyanate (“MDI”) ispreferred. A suitable commercial pMDI product is Rubinate 1840 availablefrom Huntsman, Salt Lake City, Utah, and Mondur 541 pMDI available fromBayer Corporation, North America, of Pittsburgh, Pa. Suitable commercialMUF binders are the LS 2358 and LS 2250 products from the Dyneacorporation.

The binder concentration is preferably in the range of about 1.5 wt % toabout 20 wt %, more preferably about 2 wt % to about 10 wt %. A waxadditive is commonly employed to enhance the resistance of the OSBpanels to moisture penetration. Preferred waxes are slack wax or anemulsion wax. The wax loading level is preferably in the range of about0.5 wt % to about 2.5 wt %.

After the multi-layered mats are formed according to the processdiscussed above, they are compressed under a hot press machine thatfuses and binds together the wood materials to form consolidated OSBpanels of various thickness and sizes. Preferably, the panels of theinvention are pressed for 2-10 minutes at a temperature of about 100° C.to about 260° C.

The invention will now be described in more detail with respect to thefollowing, specific, non-limiting examples.

EXAMPLES

Wood composite boards were prepared according to the present invention.Plantation-grown Loblolly pine logs (for making low density woodstrands) and natural short leaf pine logs (for making non-low densitywood strands) were obtained for use. The logs were then cut into strandsof between 1 to 6 inches in length, 0.25 to 4 inches wide and about0.025 inch thick. The strands were then dried and sorted and pressedinto six different types of panels, with the strands oriented in asingle direction, each with different combinations of strands asfollows:

TABLE I Loblolly/short leaf # Material pine (wt. %) 1 Short leafsurface, loblolly core 70/30 2 Short leaf surface, loblolly core 30/70 3Short leaf core, loblolly surface 70/30 4 Short leaf core, loblollysurface 30/70 5 Mixed throughout 70/30 6 Mixed throughout 30/70

The % listed above the weight % of the species of strands used and theirlocation in the panel. For example, panel number 1 uses 70 wt % of shortleaf pine in the surface and 30 wt % loblolly in the core. In thisinstance, the layers are defined by the wood species used in that layeras opposed to an orientation of the strands within that layer.

(As listed in the tables above, the short leaf pine is actually 70%short leaf and 20% loblolly while the loblolly is 100% loblolly.) PanelsNo. 1-2 represent panels prepared according to the present invention,while panels no. 5-6 essentially represent the prior art. The strandswere oriented in a single direction only (i.e., the core was oriented inthe same direction as the surfaces). The panels in the above examplescontained 5 wt % of Mondur G541 pMDI available from the BayerCorporation, Pittsburgh, Pa., and emulsion wax was added at a level of2% by weight of the wood with the wax solids at 58% of total.

The panels were then cut into smaller sizes and tested for severaldifferent wood composite performance characteristics according to theprotocol specified in ASTM D1037. These performance characteristicsincluded Modulus of Elasticity (“MOE”, a measure of panel stiffness) inthe parallel directions and Modulus of Rupture (“MOR”, a measure ofpanel strength) in the parallel direction. The performancecharacteristics measured for both the prior art panels and the panels ofthe present invention are set forth in table II, below.

TABLE II Panels Mixture type MOE MOR 1–2 Short leaf surface, 11858138125.0 loblolly core 3–4 Loblolly surface, 1063099 7524.5 short leafcore 5–6 Mixed Short leaf 1084803 7641.5 and loblolly

As can be seen in Table II, the OSB board prepared according to thepresent invention had significantly better performance characteristicsthan other boards, with the MOE and the MOR being significantly betterthan the prior art comparative boards.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A wood composite material comprising: wood strands disposed insurface layers and one or more core layers, whereby an intermediatedensity divides the wood strands so that the two surface layers ofstrands are more dense than the intermediate density, and the one ormore core layers of the strands are less dense than the intermediatedensity, and wherein the core layer contains at least about 70 wt % ofthe strands that are less dense than the intermediate density; and thesurface layers contain at least about 70 wt % of the strands that aremore dense than the intermediate density.
 2. The wood composite materialaccording to claim 1, wherein the wood composite board has a density ofabout 20 lbs/ft³ to about 60 lbs/ft³.
 3. The wood composite materialaccording to claim 1, wherein the strands in the one or more core layersare selected from the group consisting of oak species, hickory species,pine species, aspen species, and other hardwood and softwood species. 4.The wood composite material according to claim 1, wherein the woodcomposite material is in the form of an oriented strand board.
 5. Thewood composite material according to claim 1, wherein the wood compositematerial comprises from about 1 wt % to about 20 wt % of polymericbinders.
 6. The wood composite material according to claim 6, whereinthe wood composite board has a density of about 20 lbs/ft³ to about 40lbs/ft³.
 7. A method for producing a wood composite material comprisingthe steps of: providing a quantity of wood strands; dividing the woodstrands, based on an intermediate density, so that a first group of thestrands are less dense than the intermediate density, and a second groupof the strands are more dense than the intermediate density;constructing a wood composite material having two surface layers and oneor more core layers, wherein the core layer contains at least about 70wt % of the first group of the strands that are less dense than theintermediate density; and the surface layers contain at least about 70wt % of the second group of the strands that are more dense than theintermediate density.
 8. The method according to claim 7, furthercomprising the step of measuring the wood strands and determining anintermediate density.